On the applicability of cosine-modulated pulses for high-resolution solid-state NMR of quadrupolar nuclei with spin > 3/2.

In MQMAS-based high-resolution solid-state NMR experiments of half-integer spin quadrupolar nuclei, the high radiofrequency (RF) field requirement for the MQ excitation and conversion steps with two hard-pulses is often a sensitivity limiting factor in many practical applications. Recently, the use of two cosine-modulated (cos) low-power (lp) pulses, lasting one-rotor period each, was successfully introduced for efficient MQ excitation and conversion of spin-3/2 nuclei with a reduced RF amplitude. In this study, we extend our previous investigations of spin-3/2 nuclei to systems with higher spin values and discuss the applicability of coslp-MQ excitation and conversion in MQMAS and MQ-HETCOR experiments under slow and fast spinning conditions. For the numerical simulations and experiments we used a moderate magnetic field of 14.1 T. Two spin-5/2 nuclei (85Rb and 27Al) are mainly employed with a large variety of CQ values, but we show that the practical set up is also available for higher spin values, such as spin-9/2 with 93Nb in Cs4Nb11O30. We demonstrate for nuclei with spin value larger than 3/2 a preferential use of coslp-MQ acquisition for low-gamma nuclei and/or large CQ values with a much reduced RF-field with respect to that of hard-pulses used with conventional methods.

Indirect NMR detection via proton of nuclei subject to large anisotropic interactions, such as 14N, 195Pt, and 35Cl, using the T-HMQC sequence.

Recently, the T-hetero-nuclear multiple quantum coherence (T-HMQC) sequence using the TRAPDOR (transfer of population in double resonance) recoupling has been introduced for the indirect detection via protons of quadrupolar nuclei with spin I = 1 (14N) or 3/2 (35Cl) in solids at fast magic-angle spinning (MAS). The sequence is simple as it only uses four rectangular pulses and exhibits low t1-noise because the recoupling pulses are applied to the indirectly detected isotope, I. We demonstrate that this sequence is applicable for the detection via protons of spin-1/2 nuclei subject to large chemical shift anisotropy, such as 195Pt. We also report the proton detection of double-quantum (2Q) coherences of 14N nuclei using this sequence. This 2Q version is more robust to the adjustment of the magic angle and the instabilities of the MAS frequencies than its parent single-quantum (1Q) version since the 2Q coherences are not broadened by the first-order quadrupole interaction. In practice, than its 1Q counterpart for the indirect detection of 14N nuclei, the 2Q variant benefits from a slightly higher resolution and comparable sensitivity. In this article, we derive for the first time the Hamiltonian that describes the spin dynamics during the TRAPDOR recoupling. This Hamiltonian demonstrates the importance of the adiabaticity parameter as well as the role of third-order terms in the effective Hamiltonian. The effects of offsets, radio-frequency field, and recoupling time on the efficiency of the T-HMQC sequence are analyzed numerically as well as with experimental detection via protons of 195Pt nuclei in a mixture of cis- and trans-platin and that of 14N and 35Cl isotopes in l-histidine HCl.

Improved resolution for spin-3/2 isotopes in solids via the indirect NMR detection of triple-quantum coherences using the T-HMQC sequence.

The indirect NMR detection of quadrupolar nuclei in solids under magic-angle spinning (MAS) is possible with the through-space HMQC (heteronuclear multiple-quantum coherence) scheme incorporating the TRAPDOR (transfer of population in double-resonance) dipolar recoupling. This sequence, called T-HMQC, exhibits limited t1-noise. In this contribution, with the help of numerical simulations of spin dynamics, we show that most of the time, the fastest coherence transfer in the T-HMQC scheme is achieved when TRAPDOR recoupling employs the highest radiofrequency (rf) field compatible with the probe specifications. We also demonstrate how the indirect detection of the triple-quantum (3Q) coherences of spin-3/2 quadrupolar nuclei in solids improves the spectral resolution for these isotopes. The sequence is then called T-HMQC3. We demonstrate the gain in resolution provided by this sequence for the indirect proton detection of 35Cl nuclei in l-histidine∙HCl and l-cysteine∙HCl, as well as that of 23Na isotope in NaH2PO4. These experiments indicate that the gain in resolution depends on the relative values of the chemical and quadrupolar-induced shifts (QIS) for the different spin-3/2 species. In the case of NaH2PO4, we show that the transfer efficiency of the T-HMQC3 sequence employing an rf-field of 80 kHz with a MAS frequency of 62.5 kHz reaches 75% of that of the t1-noise eliminated (TONE) dipolar-mediated HMQC (D-HMQC) scheme.

Hadamard acquisition of 13 C-13 C 2-D correlation NMR spectra.

We show that a multiselective excitation with Hadamard encoding is a powerful tool for 2-D acquisition of 13 C─13 C homonuclear correlations. This method is not designed to improve the sensitivity, but rather to reduce the experiment time, provided there is sufficient sensitivity. Therefore, it allows fast acquisition of such 2-D spectra in labeled molecules. The technique has been demonstrated using a U─13 C─15 N histidine hydrochloride monohydrate sample allowing each point of the build-up curves of the 13 C─13 C cross-peaks to be recorded within 4 min 35 s, which is very difficult with conventional methods. Using the U─13 C─15 N f-MLF sample, we have demonstrated that the method can be applied to molecules with 14 13 C resonances with a minimum frequency separation of 240 Hz.

Efficient transfer of DNP-enhanced 1 H magnetization to half-integer quadrupolar nuclei in solids at moderate spinning rate.

We show herein how the proton magnetization enhanced by dynamic nuclear polarization (DNP) can be efficiently transferred at moderate magic-angle spinning (MAS) frequencies to half-integer quadrupolar nuclei, S ≥ 3/2, using the Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer (D-RINEPT) technique, in which a symmetry-based SR 4 1 2 recoupling scheme built from adiabatic inversion 1 H pulses reintroduces the 1 H-S dipolar couplings, while suppressing the 1 H-1 H ones. The use of adiabatic pulses also improves the robustness to offsets and radiofrequency (rf)-field inhomogeneity. Furthermore, the efficiency of the polarization transfer is further improved by using 1 H composite pulses and continuous-wave irradiations between the recoupling blocks, as well as by manipulating the S satellite transitions during the first recoupling block. Furthermore, in the case of large 1 H-S dipolar couplings, the D-RINEPT variant with two pulses on the quadrupolar channel results in an improved transfer efficiency. We compare here the performances of this new adiabatic scheme with those of its parent version with single π pulses, as well as with those of PRESTO and CPMAS transfers. This comparison is performed using simulations as well as DNP-enhanced 27 Al, 95 Mo, and 17 O NMR experiments on isotopically unmodified γ-alumina, hydrated titania-supported MoO3 , Mg(OH)2 , and l-histidine·HCl·H2 O. The introduced RINEPT method outperforms the existing methods, both in terms of efficiency and robustness to rf-field inhomogeneity and offset.

Observation of Low-γ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy.

We introduce a novel NMR approach that extends the capabilities of indirect dynamic nuclear polarization (DNP) under magic-angle spinning to probe the local environment of half-integer spin quadrupolar nuclei. Compared to cross-polarization, this novel method based on the refocused INEPT scheme with adiabatic dipolar recoupling is easier to optimize and does not distort the quadrupolar line shapes. Furthermore, the use of this technique, instead of the PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) scheme or direct DNP, greatly improves the sensitivity of DNP-NMR for the detection of quadrupolar isotopes with small dipolar couplings to protons, including notably those located in the subsurface of inorganic materials or with low gyromagnetic ratio (γ). This technique has been applied to identify the atomic-level structure of Brønsted acid sites of hydrated titania-supported MoO3, MoO3/TiO2, a widely used heterogeneous catalyst. The spectra of protonated and unprotonated 17O sites, acquired in natural abundance, indicate the presence of various oxomolybdate species as well as HOMo2 and HOMo3 Brønsted acid sites. The enhanced sensitivity of this new method has also enabled the acquisition of the first DNP-enhanced spectra of 95Mo and 47,49Ti low-γ quadrupolar isotopes. This possibility has been demonstrated by detecting the signals of these nuclei near the surface of MoO3/TiO2. This technique has allowed the observation of 49Ti surface sites, which are absent from the bulk region of TiO2. Furthermore, both 95Mo and 47,49Ti DNP spectra have shown an increased structural disorder of TiO2 and MoO3 phases near the surface of the particles and notably the preferential location of the amorphous TiO2 phase at the surface of the particles. The proposed polarization transfer is also employed to acquire the first DNP-enhanced spectrum of 67Zn, another low-γ quadrupolar isotope. This possibility is demonstrated for Al-doped ZnO nanoparticles used in optoelectronic devices. The obtained 17O, 27Al, and 67Zn DNP-NMR data prove that the surface region of these nanoparticles contains ZnO phase as well as secondary phases, such as α-Al2O3 and partially inverse ZnAl2O4 spinel.

Understanding Molecular Interactions in Rafoxanide-Povidone Amorphous Solid Dispersions from Ultrafast Magic Angle Spinning NMR.

In solid dosage formulations, probing intermolecular interactions between active pharmaceutical ingredients (APIs) and polymeric excipients, which have a mechanistic impact on physical stability as well as bioavailability, remains a challenge. In recent years, solid-state NMR spectroscopy has been demonstrated to be a powerful tool to provide structural details with an atomic resolution of therapeutic organic compounds and formulation products. However, conventional 13C-detected techniques often suffer from poor resolution and low sensitivity due to the disordered structure of certain materials such as amorphous pharmaceuticals and 13C natural abundance, hindering in-depth investigations. In this study, we utilize the magic angle spinning (MAS) technique with ultrafast speeds (UF-MAS: νR = 60 and 110 kHz) and demonstrate the enabled methods with 1H detection to study the amorphous molecular complex of rafoxanide and povidone in the solid state. The downfield shift of the RAF amide proton, resolved under UF-MAS, and its correlations with aliphatic protons of PVP, serve as strong evidence of the existence of intermolecular hydrogen bonding. Two-dimensional (2D) 1H-detected 1H{13C} and 1H-1H correlation experiments, interestingly, exhibit distinct API-polymer interactions in the spray-dried amorphous solid dispersions (ASDs), utilizing aqueous and organic cosolvents and organic solvents mixtures. The rich intermolecular interactions in the aqueously prepared ASDs presumably contribute to the physical stability, and the interactions are retained in the solution state to maintain supersaturation for an enhanced dissolution profile. This study presents the first application of UF-MAS NMR characterization of therapeutic solid dosages at a spinning frequency of 110 kHz and uncovers the molecular mechanisms of solvent-mediated pharmaceutical dispersions.

Molecular packing of pharmaceuticals analyzed with paramagnetic relaxation enhancement and ultrafast magic angle pinning NMR.

Understanding the relationship between the structure and the physicochemical attributes of crystalline pharmaceuticals requires high-resolution molecular details. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is an indispensable tool for analyzing molecular structures, but often experiences challenges of low spectral resolution and sensitivity, particularly in the characterization of unlabeled pharmaceutical materials. Besides, the relatively long spin-lattice relaxation times in pharmaceutical crystals result in time-consuming data collections. In this study, we utilize ultrafast magic angle spinning (UF-MAS) of the sample at 60 and 110 kHz to enable proton and fluorine spectroscopies for probing the structural details of crystalline posaconazole. Paramagnetic relaxation enhancement (PRE), obtained by doping Cu(ii) ions into the crystalline lattice and coating on particle surface, is implemented to shorten the spin-lattice relaxation time for speeding up the ssNMR acquisition. Our results demonstrate a remarkably improved 1H and 19F resolution and sensitivity, which enables multi-dimensional 1H-1H and heteronuclear 1H-19F correlations. In combination with density functional theory (DFT) calculations of chemical shifts, molecular details of posaconazole are established in terms of 1H and 19F networks for identifying "head-to-tail" and "head-to-head" intermolecular packings, with presumably critical contacts that stabilize the crystalline structure. The PRE and UF-MAS techniques enable the high-resolution structure characterization of fluorinated drug molecules in pharmaceutical formulations at natural abundance.

Accelerating high-resolution NMR of half-integer quadrupolar nuclei in solids: SPAM-MQMAS and SPAM-STMAS.

In solid-state NMR, multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments are well-established techniques to obtain high-resolution spectra of half-integer quadrupolar nuclei. In 2004 and 2005, a soft-pulse-added-mixing (SPAM) concept was introduced by Gan and Amoureux to enhance the S/N ratio of MQMAS and STMAS experiments. Despite their robustness and simplicity, SPAM approaches have not yet been widely applied. Here, we further exploit SPAM concepts for sensitivity enhancement upon acquisition of two-dimensional MQMAS and STMAS spectra and also establish a general procedure upon implementation of SPAM-MQMAS and SPAM-STMAS NMR. Its effectiveness and ease in experimental setup are demonstrated using simulations and experiments performed on I â€‹= â€‹3/2 (23Na, 87Rb), 5/2 (27Al, 85Rb) and 9/2 (93Nb) nuclei with a variety of quadrupolar coupling constants (CQ). Compared to the conventional z-filter methods, sensitivity enhancements in between 2 and 4 are achievable with SPAM. We recommend to use SPAM with a ratio of 4:1 for the number of echoes and antiechoes to safely maximize the sensitivity and resolution simultaneously. In addition, a comparison of the experimental approaches is made in the context of SPAM-MQMAS and SPAM-STMAS NMR with respect to repetition delay and spinning frequency, aiming to discuss the precautions upon making a judicious choice of high-resolution NMR methods of half-integer quadrupolar nuclei.

Improved sensitivity and quantification for 29Si NMR experiments on solids using UDEFT (Uniform Driven Equilibrium Fourier Transform).

We demonstrate the possibility to use UDEFT (Uniform Driven Equilibrium Fourier Transform) technique in order to improve the sensitivity and the quantification of one-dimensional 29Si NMR experiments under magic-angle spinning (MAS). We derive an analytical expression of the signal-to-noise ratios of UDEFT and single-pulse (SP) experiments subsuming the contributions of transient and steady-state regimes. Using numerical spin dynamics simulations and experiments on 29Si-enriched amorphous silica and borosilicate glass, we show that 59180298059180 refocusing composite π-pulse and the adiabatic inversion using tanh/tan modulation improve the robustness of UDEFT technique to rf-inhomogeneity, offset, and chemical shift anisotropy. These pulses combined with a two-step phase cycle limit the pulse imperfections and the artifacts produced by stimulated echoes. The sensitivity of SP, UDEFT and CPMG (Carr-Purcell-Meiboom-Gill) techniques are experimentally compared on functionalized and non-functionalized mesoporous silica. Furthermore, experiments on a flame retardant material prove that UDEFT technique provides a better quantification of 29Si sites with higher sensitivity than SP method.

Analysis of HMQC experiments applied to a spin ½ nucleus subject to very large CSA.

The acquisition of solid-state NMR spectra of "heavy" spin I = 1/2 nuclei, such as 119Sn, 195Pt, 199Hg or 207Pb can often prove challenging due to the presence of large chemical shift anisotropy (CSA), which can cause significant broadening of spectral lines. However, previous publications have shown that well-resolved spectra can be obtained via inverse 1H detection using HMQC experiments in combination with fast magic angle spinning. In this work, the efficiencies of different 195Pt excitation schemes are analyzed using SIMPSON numerical simulations and experiments performed on cis- and transplatin samples. These schemes include: hard pulses (HP), selective long pulses (SLP) and rotor-synchronized DANTE trains of pulses. The results show that for spectra of species with very large CSA, HP is little efficient, but that both DANTE and SLP provide efficient excitation profiles over a wide range of CSA values. In particular, it is revealed that the SLP scheme is highly robust to offset, pulse amplitude and length, and is simple to set up. These factors make SLP ideally suited to widespread use by "non-experts" for carrying out analyses of materials containing "heavy" spin I = 1/2 nuclei that are subject to very large CSAs. Finally, the existence of an "intermediate" excitation regime, with an rf-field strength in between those of HP and SLP, which is effective for large CSA, is demonstrated. It must be noted that in some samples, multiple sites may exist with very different CSAs. This is the case for 195Pt species with either square-planar or octahedral structures, with large or small CSA, respectively. These two types of CSAs can only be excited simultaneously with DANTE trains, which scale up the effective rf-field. Another way to obtain all the information is to perform two different experiments: one with SLP and the second with HP to excite the sites with moderate/large and small/moderate CSAs, respectively. These two complementary experiments, recorded with two different spinning speeds, can also be used to discriminate the center-band resonances from the spinning sidebands.

Recent developments in MAS DNP-NMR of materials.

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B0 ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B0 field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Forcing the 'lazy' protons to work.

The combination of cross-polarization (CP) with flip-back (FB) pulse has enabled in NMR the enhancement of 13C sensitivity and the decrease of the recycling delay at both moderate and fast magic-angle spinning (MAS) frequencies. However, only continuous-wave (CW) decoupling is presently compatible with FB-pulse (FB-CW), and depending on the CW radio-frequency (rf) field, either an insignificant sensitivity gain or an acquisition time-dependent gain and a low 13C resolution are obtained. In this study, we propose a new FB-pulse method in which radio frequency-driven recoupling (RFDR) is used as the 1H-13C decoupling scheme to overcome these drawbacks. The performances of FB-RFDR in terms of decoupling efficiency and sensitivity gain are tested on both natural abundance (NA) and uniformly 13C-15N labeled l-histidine·HCl·H2O (Hist) samples at a MAS frequency of νR = 70 kHz. The results show the superiority of RFDR over the CW decoupling with respect to these criteria. Importantly, they reveal that the sensitivity gain offered by FB-RFDR is nearly independent of the decoupling/acquisition duration. The application of FB-RFDR on NA-Hist and sucrose yields a sensitivity gain between 60 and 100% compared to conventional FB-CW and CPMAS-SPINAL experiments. Moreover, we compare the 13C sensitivities of NA-Hist obtained by our 1D FB-RFDR method and 2D 1H-{13C} double-CP acquisition. Both methods provide similar 13C sensitivity and are complementary. Indeed, the 2D method has the advantage of also providing the 1H-13C spatial proximities, but its sensitivity for quaternary carbons is limited; whereas our 1D FB-RFDR method is more independent of the type of carbon, and can provide a 13C 1D spectrum in a shorter experimental time. We also test the feasibility of FB-RFDR at a moderate frequency of νR = 20 kHz, but the experimental results demonstrate a poor resolution as well as a negligible sensitivity gain.

14N overtone nuclear magnetic resonance of rotating solids.

By irradiating and observing at twice the 14N Larmor frequency, overtone (OT) nuclear magnetic resonance (NMR) is capable of obtaining 14NOT spectra without first-order quadrupolar broadening. Direct excitation and detection of the usually "forbidden" double-quantum transition is mediated by the perturbation from the large quadrupole interaction to the spin states quantized by the Zeeman interaction. A recent study [L. A. O'Dell and C. I. Ratcliffe, Chem. Phys. Lett. 514, 168 (2011)] has shown that 14NOT NMR under magic-angle spinning (MAS) can yield high-resolution spectra with typical second-order quadrupolar line shapes allowing the measurement of 14N chemical shift and quadrupolar coupling parameters. This article has also shown that under MAS the main 14NOT peak is shifted by twice the sample spinning frequency with respect to its static position. We present the theory of 14NOT NMR of static or rotating samples and the physical picture of the intriguing spinning-induced shift in the second case. We use perturbation theory for the case of static samples and Floquet theory for rotating samples. In both cases, the results can be described by a so-called OT parameter that scales down the 14NOT radio-frequency (rf) excitation and signal detection. This OT parameter shows that the components of the rf field, which are transverse and longitudinal with respect to the magnetic field, are both effective for 14NOTrf excitation and signal detection. In the case of MAS at angular frequency ωr , the superposition of the excitation and detection components in the OT parameter makes either the +2ωr or -2ωr term the dominant 14NOT signal, depending on the sense of sample spinning with respect to the magnetic field. This leads to an apparent 14NOT signal shifted at twice the spinning frequency. The features of 14NOT NMR spectra for both static and rotating samples are illustrated with simulations. The spinning induced shift and its dependence on the spinning direction are confirmed experimentally by reversing the spinning direction and the field of the 36 T series-connected hybrid magnet at the US National High Magnetic Field Laboratory.

Observation of proximities between spin-1/2 and quadrupolar nuclei in solids: Improved robustness to chemical shielding using adiabatic symmetry-based recoupling.

We introduce a novel heteronuclear dipolar recoupling based on the R21-1 symmetry, which uses the tanh/tan (tt) shaped pulse as a basic inversion element and is denoted R21-1(tt). Using first-order average Hamiltonian theory, we show that this sequence is non-γ-encoded and that it reintroduces the |m| = 1 spatial component of the Chemical Shift Anisotropy (CSA) of the irradiated isotope and its heteronuclear dipolar interactions. Using numerical simulations and one-dimensional (1D) 27Al-{31P} through-space D-HMQC (Dipolar Heteronuclear Multiple-Quantum Correlation) experiments on VPI-5, we compare the performances of this recoupling to those of other non-γ-encoded |m| = 1 heteronuclear recoupling schemes: REDOR (Rotational-Echo DOuble Resonance), SFAM (Simultaneous Frequency and Amplitude Modulation) and R42-1(tt). Such comparison indicates that the R21-1(tt) scheme is more robust to CSA, offset and radiofrequency field inhomogeneities than the other schemes. We take advantage of the high robustness of R21-1(tt) to CSA and offset to demonstrate the possibility to correlate the signals of 207Pb isotope with those of neighboring half-integer spin quadrupolar nuclei. Such approach is demonstrated experimentally by acquiring 11B-{207Pb} D-HMQC 2D spectra of Pb4O(BO3)2 crystalline powder.

Solid-State NMR Spectroscopy Proves the Presence of Penta-coordinated Sc Sites in MIL-100(Sc).

Advanced solid-state NMR methods and first-principles calculations demonstrate for the first time the formation of penta-coordinated scandium sites. These coordinatively unsaturated sites were shown during the thermal activation of scandium-based metal-organic frameworks (MOFs). A 45 Sc NMR experiment allows their specific observation in activated Sc3 BTB2 (H3 BTB=1,3,5-tris(4-carboxyphenyl)benzene) and MIL-100(Sc) MOFs. The assignment of the ScO5 groups is supported by the DFT calculations of NMR parameters. The presence of ScO5 Lewis acid sites in MIL-100(Sc) explains furthermore its catalytic activity. The first NMR experiment to probe 13 C-45 Sc distances is also introduced. This advanced solid-state NMR pulse sequence allows the demonstration of the shrinkage of the MIL-100(Sc) network when the activation temperature is raised.

Correction: Analysis of local molecular motions of aromatic sidechains in proteins by 2D and 3D fast MAS NMR spectroscopy and quantum mechanical calculations.

Correction for 'Analysis of local molecular motions of aromatic sidechains in proteins by 2D and 3D fast MAS NMR spectroscopy and quantum mechanical calculations' by Piotr Paluch et al., Phys. Chem. Chem. Phys., 2015, 17, 28789-28801.

1H-31P CPVC NMR method under Very Fast Magic Angle Spinning for analysis of dipolar interactions and dynamics processes in the crystalline phosphonium tetrafluoroborate salts.

We present an NMR methodology which can be used to study the dynamical processes occurring in organophosphorus compounds that belong to the group of the organic ionic plastic crystals (OIPCs). As model samples we employed two phosphonium tetrafluoroborate salts; (t-Bu)3PH+BF4- (1) and (Me)3PH+BF4- (2). Both samples possess in their structures direct H-P bonds, and both undergo complex thermal processes in the solid state, forming below the melting point three or four phases, respectively. 1H-31P CPVC (Cross-Polarization Variable Contact) measurements were performed under Very Fast Magic Angle Spinning with speed equal to 50 or 60 kHz, in order (i) to establish the hydrogen-phosphorus dipolar couplings, and (ii) to correlate the dipolar splitting values with molecular motions of the cation. Our project is divided into three sections. In the first part we present DSC studies of (1) and (2), to verify whether these samples fulfill the requirements that define them as OIPC. The second part is dedicated to a discussion of the theoretical aspects of 1H-31P CPVC and especially an analysis of the influence of different parameters, e.g. CSA31P, H-H mismatch, rf-inhomogeneity, dipolar truncation, and the type of dynamics through the motionally averaged <ηD> asymmetry value on the NMR response. The third part shows experimental 1H-31P CPVC data and applicability of these measurements to study H-P distances and dynamics. The complex molecular motion for sample (2), including rotation and diffusion, versus temperature is then postulated on the bases of the changes of H-P dipolar splitting.

3D correlation NMR spectrum between three distinct heteronuclei for the characterization of inorganic samples: Application on sodium alumino-phosphate materials.

We report here an original NMR sequence allowing the acquisition of 3D correlation NMR spectra between three distinct heteronuclei, among which two are half-integer spin quadrupolar nuclei. Furthermore, as two of them exhibit close Larmor frequency, this experiment was acquired using a standard triple-resonance probe equipped with a commercial frequency splitter. This NMR technique was tested and applied to sodium alumino-phosphate compounds with 31P as the spin-1/2 nucleus and 23Na and 27Al as the close Larmor frequencies isotopes. To the best of our knowledge, such experiment with direct 31P and indirect 27Al and 23Na detection is the first example of 3D NMR experiment in solids involving three distinct heteronuclei. This sequence has first been demonstrated on a mixture of Al(PO3)3 and NaAlP2O7 crystalline phases, for which a selective observation of NaAlP2O7 is possible through the 3D map edition. This 3D correlation experiment is then applied to characterize mixing and phase segregation in a partially devitrified glass that has been proposed as a material for the sequestration of radioactive waste. The 31P-{23Na,27Al} 3D experiment conducted on the partially devitrified glass material conclusively demonstrates that the amorphous component of the material does not contain aluminum. The as-synthesized material thus presents a poor resistance against water, which is a severe limitation for its application in the radioactive waste encapsulation domain.

Imaging the spatial distribution of radiofrequency field, sample and temperature in MAS NMR rotor.

We investigate using nutation experiments the spatial distribution of radiofrequency (rf) field, sample, temperature and cross-polarization transfer efficiency in 1.3 mm rotor. First, two-dimensional (2D) 1H nutation experiments on silicone thin cylinders in the presence of B0 field gradient generated by shim coils are used to image the spatial distribution of rf field inside the rotor. These experiments show that the rf field is asymmetrical with respect to the center of the rotor. Moreover, they show the large inhomogeneity that still remains across the sample when using spacers, and that even in this case, the rf-field close to the drive cap is decreased to ca. only 20% of its maximum value. Such 2D nutation experiment in the presence of B0 field gradient are also employed to demonstrate the migration of adamantane sample from the center of the rotor to its ends during Magic-Angle Spinning (MAS). Furthermore, 2D 1H nutation experiments on nickelocene exhibiting temperature-dependent isotropic chemical shift provides insights into the temperature distribution inside rotor. Finally three-dimensional (3D) 1H → 13C Cross-Polarization under MAS (CPMAS) nutation experiment indicates that only nuclei subject to the largest rf field contribute to the CPMAS transfer, when using rf field of constant amplitude on both channels. Such high selectivity allows the determination of accurate dipolar coupling constants in the Cross-Polarization with Variable Contact (CP-VC) experiment under fast MAS, at the expense of low sensitivity. Conversely when using ramped-amplitude on the 1H channel during the CPMAS transfer, nuclei subject to smaller rf field contributes to the transfer, which increases the sensitivity of CPMAS experiment but does not allow an accurate determination of dipolar coupling constants using CP-VC experiment.